1. Field of the Invention
The present invention relates to a support device for a chair which enables the height of the chair to be adjusted for a typist or other swivel chair.
2. Description of the Prior Art
Adjustable support devices for swivel chairs conventionally comprise a screw lift assembly. In these devices the height of the chair is adjusted by rotating the chair about its swivel axis or by rotating another component. Support devices which enable the height of the chair to be adjusted without having to rotate the chair or another component, have been proposed in the form of a so-called gas spring consisting of a pneumatic cylinder which can support the chair at a selected height. The gas, usually nitrogen, is sealed within the cylinder but in practice it is almost impossible to achieve a perfect seal with the result that the cylinder does tend to leak, albeit at a very slow rate. Typically, the leakage is such that the cylinder will cease to function correctly after about two years use.